In general, a lithographic printing plate is composed of an oleophilic image area accepting ink and a hydrophilic non-image area accepting dampening water in the process of printing. Lithographic printing is a printing method utilizing the nature of water and oily ink to repel with each other and comprising rendering the oleophilic image area of the lithographic printing plate to an ink-receptive area and the hydrophilic non-image area thereof to a dampening water-receptive area (ink-unreceptive area), thereby making a difference in adherence of the ink on the surface of the lithographic printing plate, depositing the ink only to the image area, and then transferring the ink to a printing material, for example, paper.
In order to produce the lithographic printing plate, a lithographic printing plate precursor (PS plate) comprising a hydrophilic support having provided thereon an oleophilic photosensitive resin layer (image-recording layer) has heretofore been broadly used. Ordinarily, the lithographic printing plate is obtained by conducting plate making according to a method of exposing the lithographic printing plate precursor through an original, for example, a lith film, and then while leaving the image-recording layer corresponding to the image area, removing the unnecessary image-recording layer corresponding to the non-image area by dissolving with an alkaline developer or a developer containing an organic solvent thereby revealing the hydrophilic surface of support.
In the hitherto known plate making process of lithographic printing plate precursor, after exposure, the step of removing the unnecessary image-recording layer by dissolving, for example, with a developer is required. However, it is one of the subjects to save or simplify such an additional wet treatment described above. Particularly, since disposal of liquid wastes discharged accompanying the wet treatment has become a great concern throughout the field of industry in view of the consideration for global environment in recent years, the demand for the solution of the above-described subject has been increased more and more.
As one of simple plate making methods in response to the above-described requirement, a method referred to as on-machine development has been proposed wherein a lithographic printing plate precursor having an image-recording layer capable of being removed in the unnecessary areas during a conventional printing process is used and after exposure, the unnecessary area of the image-recording layer is removed on a printing machine to prepare a lithographic printing plate.
Specific methods of the on-machine development include, for example, a method of using a lithographic printing plate precursor having an image-recording layer that can be dissolved or dispersed in dampening water, an ink solvent or an emulsion of dampening water and ink, a method of mechanically removing an image-recording layer by contact with rollers or a blanket cylinder of a printing machine, and a method of lowering cohesion of an image-recording layer or adhesion between an image-recording layer and a support upon penetration of dampening water, ink solvent or the like and then mechanically removing the image-recording layer by contact with rollers or a blanket cylinder of a printing machine.
Also, as another example of simple plate making methods, a method referred to as gum development has been proposed wherein removal of the unnecessary area of the image-recording layer in the development processing step is carried out using a gum solution for finishing treatment which is conventionally performed after an alkali development in place of a conventional highly alkaline developer.
In the invention, unless otherwise indicated particularly, the term “development processing step” means a step of using an apparatus (ordinarily, an automatic developing machine) other than a printing machine and removing an unexposed area in an image-recording layer of a lithographic printing plate precursor upon contact with liquid (ordinarily, an alkaline developer) thereby revealing a hydrophilic surface of support. The term “on-machine development” means a method or a step of removing an unexposed area in an image-recording layer of a lithographic printing plate precursor upon contact with liquid (ordinarily, printing ink and/or dampening water) by using a printing machine thereby revealing a hydrophilic surface of support.
Of the processings including the “development processing step”, the development using a gum solution as the developer is particularly referred to as “gum development”.
On the other hand, digitalized technique of electronically processing, accumulating and outputting image information using a computer has been popularized in recent years, and various new image-outputting systems responding to the digitalized technique have been put into practical use. Correspondingly, attention has been drawn to a computer-to-plate technique of carrying digitalized image information on highly converging radiation, for example, a laser beam and conducting scanning exposure of a lithographic printing plate precursor with the radiation thereby directly preparing a lithographic printing plate without using a lith film. Thus, it is one of the important technical subjects to obtain a lithographic printing plate precursor adaptable to the technique described above.
In the simplification of plate making operation and the realization of dry system or non-processing system as described above, since the image-recording layer after the exposure is not fixed with the development processing, it is still sensitive to light and likely to be fogged before printing. Therefore, an image-recording layer capable of being handled in a bright room or under a yellow lump and a light source are necessary. As such a laser light source, a semiconductor laser emitting an infrared ray having a wavelength of 760 to 1,200 and a solid laser, for example, YAG laser, are extremely useful because these lasers having a large output and a small size are inexpensively available. Also, an UV laser can be used.
As the lithographic printing plate precursor of on-machine development type capable of conducting image-recording with an infrared laser, for example, lithographic printing plate precursors having provided on a hydrophilic support, an image-forming layer (image-recording layer) in which hydrophobic thermoplastic polymer particles are dispersed in a hydrophilic binder are described in Japanese Patent 2,938,397 (corresponding to EP0770494A2). It is described in Japanese Patent 2,938,397 (corresponding to EP0770494A2) that the lithographic printing plate precursor is exposed to an infrared laser to agglomerate the hydrophobic thermoplastic polymer particles by heat thereby forming an image, and mounted on a plate cylinder of a printing machine to be able to carry out on-machine development by supplying dampening water and/or ink.
Although the method of forming image by the agglomeration of fine particles only upon thermal fusion shows good on-machine development property, it has a problem in that the image strength is extremely weak and printing durability is insufficient.
Further, lithographic printing plate precursors having provided on a hydrophilic support, microcapsules containing a polymerizable compound encapsulated therein are described in JP-A-2001-277740 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”) and JP-A-2001-277742.
Moreover, lithographic printing plate precursors having provided on a support, a photosensitive layer containing an infrared absorbing agent, a radical polymerization initiator and a polymerizable compound are described in JP-A-2002-287334 (corresponding to US2002/0177074A1).
The methods using the polymerization reaction as described above have a feature that since the chemical bond density in the image area is high, the image strength is relatively good in comparison with the image area formed by the thermal fusion of fine polymer particles. However, it is necessary to provide an intermediate layer between the support and the image-recording layer in order to satisfy the printing durability, on-machine development property and stain resistance at the time of printing (staining property).
It is ordinarily known that a water-soluble resin imparted with a hydrophilicity is used in the intermediate layer of such a purpose and lithographic printing plate precursors of on-machine development type provided with an intermediate layer composed of a polymer having an adsorbing group to substrate, a polymerizable group and a hydrophilic group are described in JP-A-2005-125749 (corresponding to US2005/0074692A1).
However, with respect to the lithographic printing plate precursor of on-machine development type and lithographic printing plate precursor of gum development type, even when such an intermediate layer is provided, the staining property of the non-image area is still insufficient, in particular, a round spot-like stain (hereinafter also referred to as a spot stain) of several mμ to several thousands mμ may occur in some cases and therefore, a further improvement has been requested.